Controlled dispersion of injected fuel

ABSTRACT

An internal combustion engine having a cylinder head at one end of the cylinder, a piston mounted for reciprocation in the cylinder, a cavity in the cylinder head, and a fuel injector nozzle located in the cavity in the cylinder head to deliver fuel into the cavity in a direction towards the piston. The piston has a bowl in the top thereof, positioned to intersect the path of delivery of the fuel from the injector nozzle. A rim extends around the periphery of the bowl and is upstanding from the to surface of the piston. The bowl and rim effect containment of the fuel, particularly during high fueling rates, to control the combustion process.

This invention relates to the combustion process in internal combustionengines particularly two stroke cycle engines and specifically relatesto the fuel/air preparation within the engine and distribution of fueland air mixture in the combustion chamber.

In the control of contaminates in the exhaust gas of an internalcombustion engine, it is desirable to obtain effective distribution ofthe fuel in the dilutant charge within the cylinder, the dilutant chargebeing made up of air or a mixture of air and retained exhaust gas.However, in the preparation of the combustible charge, by theintroduction of fuel into the dilutant charge, it is desirable toprevent the fuel being distributed to parts of the combustion chamberdistant from the point of ignition Wide distribution of the fuel in thedilutant charge creates local fuel charge mixtures which are lean andtherefore difficult to ignite and/or maintain the combustion.Combustible lean mixtures promote the production of NOx, as combustionis taking place in an oxidant rich mixture, and non-combustible leanmixtures lead to the emissions of hydrocarbons in the form of unburnedfuel in the exhaust gas.

The distribution of the fuel in the combustion chamber is of particularimportance under low engine load conditions when the quantity of fuelbeing delivered per cycle is relatively small and therefore thedistribution thereof must be contained in order to establish aneffectively ignitable and combustible mixture. However under high engineloads it is desirable to burn up substantially all oxidant in thedilutant charge which may be achieved by obtaining a greater degree ofdistribution of the fuel within the dilutant charge so that the fuel isexposed to sufficient oxidant providing combustible fuel chargemixtures, to maximise air utilisation and achieve effective combustionof the relatively greater quantity of fuel delivered to the engine percycle at the high engine loads.

It has previously been proposed to endeavour to control the distributionof the fuel in a premixed charged engine by controlling the movement ofthe charge upon entry into the combustion chamber so that a degree ofstratification of the fuel is obtained in the combustion chamber. Thiscontrol of the premixed charge is difficult to obtain in a two strokecycle engine as the gas movement in the combustion chamber is complex,and may experience many changes within that part of the engine cycleduring which the air and fuel mixture is entering the combustionchamber, while at the same time the exhaust gases are leaving thecombustion chamber.

There has been proposed in the applicant's prior U.S. Pat. No.4,719,880, wherein the fuel is injected directly into the enginecombustion chamber and a cavity is provided in the cylinder head withthe fuel being injected into that cavity. In that prior proposal, thefuel injector system is of a low penetration type so that the fuel issubstantially contained within the cavity in the cylinder head.

Further the cavity in the cylinder head is shaped to establish acircular motion of the charge in the cylinder so the fuel as it entersthe cavity is carried by the air motion from the injector to the pointof ignition. This combustion system limits the distribution of the fuelwithin the engine combustion chamber so that the fuel/air mixturepresented to the point of ignition is of a readily ignitable andcombustible character.

The combustion system as proposed in the above referred to U.S. Patentis particularly effective under light to medium engine load conditionswhere the fuelling rate is comparatively low. However, as the fuellingrate increases to meet higher engine loads, it is necessary to obtain agreater degree of distribution of the fuel within the combustion chamberso that the fuel is exposed to substantially all the oxidant throughoutthe combustion chamber, or at least sufficient oxidant, providingcombustible fuel charge mixtures to effect complete combustion of thefuel whilst also containing the fuel to provide a readily ignitable andcombustible charge about the point of ignition. Without increaseddistribution of the fuel under high engine load conditions the actualpower output of the engine may be restricted and the hydrocarbon contentof the exhaust gas may increase due to the incomplete combustion of allthe fuel supplied to the combustion chamber and/or incompleteutilisation of all the oxidant in the dilutant charge throughout thecombustion chamber.

It is therefore an object of the present invention to provide aninternal combustion engine wherein improved preparation of the fuel andair mixture and the distribution thereof in the combustion chamber canbe improved and controlled so as to enhance the control of the level ofemissions in the exhaust gas and the power output of the engine.

With this object in view there is provided an internal combustion enginehaving a cylinder head at one end of the cylinder, a piston mounted forreciprocation in the cylinder, a cavity in the cylinder head, a fuelinjector nozzle located in the cavity in the cylinder head to deliverfuel into the cavity in a direction towards the piston, a bowl at thetop of the piston positioned to intersect the line of the path ofdelivery of the fuel from the nozzle, whereby at high fueling rates atleast a portion of the fuel injected into the cylinder enters the bowl,and a rim around the periphery of the bowl upstanding from the topsurface of the piston.

The bowl in the top of the piston is positioned and configured so thatat least part of the fuel entering the bowl rebounds off the base of thebowl on a path directed towards the cavity in the cylinder head.Preferably, the bowl in the piston is generally aligned with the cavityin the cylinder head and more particularly so that the axial centre lineof the bowl generally aligns with the path of the fuel delivered fromthe injector located in the cavity in the cylinder head. The injector ispreferably of the type that produces a penetrating fuel spray so thatthe fuel issuing therefrom will travel a sufficient distance to enterthe bowl having regard to the position of the piston in the cylinder atthe time of injection.

It is of course, well known to provide a bowl in the top face of apiston and to direct fuel from a fuel injector in a direction to enterthat bowl. This being a particularly common practice in diesel orcompression ignition engines. The disadvantage of using a bowl in thepiston of a spark ignited engine, which is basically of a lighterconstruction than diesel engines, is that the provision of the bowlrequires the thickness of the head of the piston to be substantiallyincreased As a result the weight of the piston is increased and also thevolume of metal in the piston in which heat build up may occur. It willfurther be appreciated that the provision of a bowl in the pistonincreases the surface area exposed to the combustion gases and that thedeeper the bowl, the greater the increase in surface area and thethickness of the top of the piston, and hence the greater the mass andheat build up.

However if the bowl is comparatively shallow, thus reducing thenecessary thickness of the top of the piston, the effectiveness of thebowl in controlling the fuel dispersion is significantly reduced.

The provision of the upstanding peripheral rim about a shallow bowlcreates an eccentuated upward movement of the charge gas in the cylinderabout the periphery of the bowl as the piston moves towards the cylinderhead on the compression stroke. This upward flow of the air around theperiphery of the of the bowl provides an increase in the containment ofthe fuel within the gas charge immediately above the bowl. In this way,the control of the dispersion of the fuel is increased in a mannersimilar to that which would be achieved with a much deeper bowl whilstavoiding the disadvantages above discussed arising from the use of adeep bowl in the top of the piston.

The outer surface of the rim is preferably upwardly inclined or concavein cross section to sweep smoothly upward from the surface of the top ofthe piston. The outer surface of the tim may be inclined to the bowlaxes at between 30° and 50°. Conveniently the peripheral rim forms asignificant part of the total depth of the bowl, such as 50% or more,preferably up to 75%.

The bowl is preferably of a substantially circular cross-section with adiameter of not more than 0.5 the diameter of the cylinder, morespecifically, preferably between 0.25 and 0.35 the cylinder diameter.The total depth of the bowl is conveniently up to about 6 mm, orpreferably about 3 mm, from the top surface of the piston.

The bowl at the top of the piston is preferably substantially alignedwith the path of the fuel delivered by the nozzle so that the fuelissuing from the nozzle impinges on the base of the bowl and reboundtherefrom towards the cavity in the cylinder head.

The cavity in the cylinder head is conveniently of a configuration thatextends in a generally diametral direction from adjacent the cylinderwall at a location opposite the exhaust port, the cavity havingsubstantially straight side edges in the diametral direction of extentthereof, the longitudinal length of the cavity in said direction beingless than the diameter of the cylinder and preferably between about 0.5and 0.8 of the diameter of the cylinder.

Preferably the base of the cavity in the cylinder head is a continuoussmooth surface with the cavity having a maximum depth of about 0.25 to0.55 of the diameter of the cylinder, preferably more than 0.35 andpreferably less than 0.45. Conveniently the ratio of the length of thecavity in the direction of extension thereof to the maximum depth of thecavity is in the range of about 1 to 3, preferably below 2.5 andpreferably about 1.5.

In a preferred form the cavity has a bottom surface extending betweenthe side walls of the cavity that is a generally continuous curve in thedirection of the extension of the cavity, with the deepest partsubstantially arcuate and blending smoothly with near straight parts ateach end that extend to the face of the cylinder head. The bottomsurface of the cavity at the exhaust port end is steeply inclined to theface of the cylinder head and is preferably substantially normal to thediametral plane of the cylinder where it meets the cylinder head face.The other near straight part of the cavity bottom surface is lesssteeply inclined, the inclination resulting in the other end of thebottom surface meeting the face of the cylinder head in the vicinity ofthe upper end of the wall of the cylinder above the inlet port.

The shape of the bottom of the cavity, and particularly thesubstantially arcuate portion at the end of the cavity opposite to thatat the inlet port end, promotes the establishment of a rotating orswirling motion in the charge about an axis transverse to thelongitudinal direction of the cavity. This rotating or swirling motionof the charge is particularly advantageous under low fuelling rates asit will tend to retain the fuel within the cavity to provide effectiveignitability and combustibility of the fuel/air mixture.

In this regard it will be appreciated that under low fuelling rates,that is under low engine load conditions, the timing of the fuelinjection is normally late in the compression stroke and ignition closeto the end of the injection period. Also at this late period in thecompression stroke, a relatively high degree of swirl will have beenestablished in the cavity in the cylinder head which will promote theretention of the fuel injected under light load within the swirling air.

Under medium and high engine load conditions where the fuelling rate issubstantial, it is customary to commence injection of the fuel earlierin the compression stroke in comparison with the injection timing at lowload and low fueling rates. Thus injection of the fuel takes place whenthe rate of swirl generated by the cavity in the cylinder head is lowand thus the fuel will penetrate further into and be dispersed morewidely in the gas charge. This wider dispersion of the fuel may giverise to detonation commonly referred to as "knock" and/or may give riseto insufficient emission control.

In an engine using a flat or domed top piston with a relatively highpenetration spray, fuel from the injector that strikes the piston tendsto flow outwardly over the top surface of the piston towards thecylinder walls.

In order to avoid fuel mixing with air located in areas remote from thepoint of ignition, the bowl in the top of the piston is provided toassist in containment of the fuel.

As the piston moves towards the cylinder head, during the compressionstroke, there is an air flow established from the outer or peripheralareas of the cylinder towards the cavity in the cylinder head. Furtherduring the injection of the fuel into the combustion chamber undermedium to high load conditions the fuel will penetrate a distance downthe cylinder to impinge upon the base of the bowl in the piston. Theimpinging of the fuel on the base of the bowl will cause a rebound ofthe fuel in a generally upward direction towards the cavity in thecylinder head.

The rebound movements of the fuel and air charge combined with theoverall movement of the charge in the cylinder towards the cavity in thecylinder heat will assist in reducing the radial dispersion of the fuel.Further, the upstanding rim about the periphery of the bowl enhances theabove effect as the concave external surface of the rim promotes anupward directed flow of the air about the periphery of the bowl as thepiston moves towards the cylinder head to also contribute to thecontainment of the fuel. Thus the migration of the fuel into theperipheral areas of the cylinder where detonation is likely to occur,and where bulk quenching of the charge can cause a large increase ofhydrocarbon emission to occur, is largely prevented.

In particular, the bowl in the piston and the rim thereof upstandingfrom the top of the piston intensifies the upward movement of the aircharge in the area where the fuel is rebounded from the piston so thatthe fuel is not carried into high squish areas between the piston andcylinder head, where it can contribute to higher emissions levels, andwhere detonation can occur.

The invention will be further described with reference to one practicalarrangement of an internal combustion engine incorporating the presentinvention as illustrated in the accompanying drawings.

FIG. 1 is an axial cross-section of the engine and cylinder head;

FIG. 2 is an underneath perspective view of the cylinder head;

FIGS. 3A and 3B show a plan view and a cross-sectional view of thepiston, respectively.

Referring now to the drawings, cylinder 5 has a piston 6 disposedtherein to reciprocate in the axial direction of the cylinder andcoupled to a crank shaft not shown in the drawing. The circumferentialwall 7 of the cylinder has an exhaust port 8 and a diametricallyopposite inlet or transfer port 9 divided into two parts. A pair ofadditional transfer ports 10 and 11 are disposed generally symmetricallyon either side of the transfer port 9.

The upper end of the cylinder 5 is closed by a detachable cylinder head12 having a cavity or cavity 13 formed therein in an eccentricdisposition with respect to the cylinder axis 20. An opening is providedin the top of the cavity in which the fuel injector 14 is mounted, andan aperture 15 is provided at the inner end of the cavity for thereceipt of a conventional spark plug. The head 17 of the piston 6 isslightly domed, and the opposing underface 16 of the cylinder head 12 isof a complimentary concave shape except for the provision of the cavity13 therein.

The cavity 13 is located substantially symmetrical with respect to theaxial plane of the cylinder extending through the centre of the transferport 9 and exhaust port 8. The cavity 13 extends across the cylinderfrom the cylinder wall immediately above the transfer port 9, to adistance past the cylinder centre line 20.

As previously described the cross sectional shape of the cavity 13 alongthe axial plane of the cylinder is substantially arcuate at the base 24,with the centre line 21 of the arc somewhat closer to the centre line 20of the cylinder than the cylinder wall above the transfer port 9. Theend of the arcuate base 24 closer to the cylinder wall, above thetransfer port 9, merges with a generally straight face 25 extending tothe underface 16 of the cylinder head 12 at the cylinder wall 7. Theface 25 is inclined upwardly from the cylinder wall 7 to the arcuatebase 24 of the cavity.

The opposite or inner end of the arcuate base 24 merges with arelatively short generally vertical face 26 that extends to theunderface 16 of the cylinder head 12. The face 26 is generally parallelto the cylinder axis so that it meets the underface at a relativelysteep angle. The opposite side walls 27 and 28 of the cavity aregenerally flat and parallel to the axial plane of the cylinder, and soalso meet the underface 16 of the cylinder head at a steep angle.

The piston 6 has a top face or crown 30 of slightly domed or convexshape about the periphery as seen in FIG. 1. The bowl 31 on the pistonis formed in part by a depression in the piston crown 30, and a rim 32surrounding the depression and upstanding from the piston crown. Thebowl 31 is of circular cross-section in the direction normal to thecylinder and piston axis 20, and is offset from the piston axis towardsthe transfer or inlet port 9 side of the cylinder. This offsetdisposition of the bowl 31 locates the bowl generally aligned with thedirection of the path of the fuel as delivered by the fuel injector 14.

The internal surface 33 of the bowl is substantially cylindrical andcontiguous with the internal surface of the rim 32. The external surface34 of the rim is inward and upwardly inclined at 30° to the axis of thebowl 31 to provide a smooth transition between the generally rightanglerelated crown 30 of the piston and the rim 32. The concave shape of theexternal surface 34 also contributes significantly to the directing ofthe charge flow towards the cavity 13 during the compression stroke ofthe piston.

In a conventional flat top or low crown piston as the piston movestowards the cylinder head, and after the inlet and exhaust ports haveclosed, there will be a compression of the charge, principally air,trapped in the cylinder. This will result in some movement of the chargeinwardly from the peripheral areas of the cylinder to enter the cavity13 as the piston rises in the cylinder. However because of thecontinuous nature of the top surface of the piston, a degree ofturbulence will result in the air flow in the vicinity of the centre ofthe cylinder.

Fuel delivered by the fuel injector into this turbulent air, and/or fuelrebounded off the piston crown into the turbulent air, can be carriedtowards the peripheral area of the cylinder because of the turbulence.The high squish effects experienced in the peripheral area of thecylinder are likely to result in detonation of the fuel/air charge.Detonation is particularly likely to occur at high engine loads when thefuel rate is high and hence the probability of migration of fuel to theperipheral areas due to turbulence is high.

The provision of the bowl 31 in the piston and the upwardly swept outersurface of the rim 32 of bowl, promotes a more orderly flow of the aircharge as the piston moves towards the cylinder head. The air movinginwardly from the peripheral area of the cylinder is changed to anupward direction of flow in a more smooth and controlled manner therebyreducing the level of turbulence. As a result, the fuel is morecontained in the upwardly moving air and so migration of fuel to theperipheral area of the cylinder is reduced. Particularly during highfueling rates the fuel which is directed by the injector 14 into thebowl 31 is bounced off the bottom of the bowl in a generally upwarddirection. This rebounded fuel becomes entrained in the upwardly movingair flow coming off the external surface of the rim 32 of the bowl andcarried upwardly therewith.

The above discussed control of the air and fuel flows during thecompression stroke of the piston contributes significantly to reductionor prevention of detonation particularly at higher fuelling rates and/orcontrol of emission.

In one specific embodiment of a two stroke cycle engine incorporatingthe present invention, the bore of the cylinder is 85 mm with the cavityhaving a length in the direction through the centre of the cylinder of52 mm and a width of 43 mm. The centre line 21 of the arcuate base 24 isoffset from the centreline 20 of the cylinder by 12 mm and from theplane of the low face of the head 12 by 28 mm. The radius of the arcuatebase is 23 mm.

The piston 6 has a diameter or a length in the direction through thecentre of the cylinder of 84 mm, with the bowl 31 having a diameter orlength of 32 mm. The centre line 22 of the bowl is offset from thecentreline 20 of the piston by 11.15 mm. The bowl has a depth of 3 mmbelow the top surface of the piston and its upstanding rim has a heightof 3 mm above the surface of the piston with its external concavesurface inclined at 30° thereto.

As can be seen in FIG. 1 the configuration of the passage 30 leading tothe transfer port 9 is upwardly inclined towards the cylinder head 12 sothat the charge entering the cylinder through the transfer port 9 willbe similarly upwardly directed into the cavity 13. The transfer ports 10and 11 are similarly shaped to direct the charge upwardly towards thecavity 13 and are inclined towards the central transfer port 9 so as todirect the flow of the charge through the ports 10 and 11 inwardlytowards the cavity 13.

The general direction of the incoming charge of air is upwardly into thecavity 13 in the cylinder head, to enter at the area adjacent thecylinder wall 7. The portion of the charge entering the cavity is formedinto a rotating or swirling motion by the arcuate shape of the base ofthe cavity. The upward movement of the incoming charge on the left handside of the cylinder, as seen in FIG. 1, displaces the exhaust gasesfrom the previous cycle towards the right hand side of the cylinder andhence towards the exhaust port 8. In addition the circular motion of theincoming charge entering the cavity 13 scavenges the exhaust gases fromthe cavity and promotes the flow thereof towards the exhaust port 8.

The initial flow of charge through the transfer ports into the cylinder,whilst the piston is displaced a considerable distance down thecylinder, will create an initial rotary or swirling motion of the chargein the cavity 13 and the adjacent area of the cylinder. As the pistonmoves up the cylinder the air in the cylinder will be displaced towardsthe cavity to be entrained in the swirling charge. It will also contractthe charge into a smaller volume and so increase the speed of rotationof the charge in the cavity.

The steep inclination of the face 26 of the inner end wall and oppositeside walls 27 and 28 of the cavity 13 assists in retaining the rotatingcharge cloud in the cavity. Also as the piston rises in the cylinder,the charge trapped between the piston crown and cylinder head movestoward and into the cavity 13 over these steep walls. This chargemovement contributes to the maintenance of an oxidant rich cloud in thecavity to support combustion of the fuel therein.

Further information regarding the characteristics of the cavity 13 inthe cylinder head and the performance thereof under low fuelling ratesare disclosed in U.S. Pat. No. 4,719,880 and the disclosure therein isincorporated herein by reference.

A fuel metering and injection method and device suitable for meteringand delivering fuel to an engine operating as disclosed herein aredescribed in detail in U.S. Pat. No. 4,693,224, the disclosure of whichis hereby incorporated by reference for the teaching of fuel meteringand injection method and device therein.

The operation and construction of an internal combustion engine asherein disclosed by be used in many forms of two stroke cycle engineincluding for or incorporated in vehicles for use on land, sea or in theair including engines in or for motor vehicles, boats or aeroplanes. Inparticular the engines as herein described may be installed in a boat,vehicle or aeroplane to propel same, and includes outboard marineengines.

We claim:
 1. An internal combustion engine, comprising:a cylinder; acylinder head at one end of the cylinder; a piston mounted forreciprocation in the cylinder; a cavity in the cylinder head; a fuelinjector nozzle located in the cavity in the cylinder head to deliverfuel in a direction towards the piston, the piston having a top facedirected toward the cylinder head; a bowl in the top face of the pistonpositioned so that throughout at least a final half of the compressionstroke of the piston, the bowl is located in a path of delivery of thefuel from the nozzle so that fuel will be delivered into the bowl, and arim extends about the periphery of the bowl upstanding from the topsurface of the piston, wherein an external peripheral surface of the rimis inclined upwardly and inwardly at an angle of about 45° to the axisof the bowl.
 2. An internal combustion engine, comprising:a cylinder; acylinder head at one end of the cylinder; a piston mounted forreciprocation in the cylinder; a cavity in the cylinder head; a fuelinjector nozzle located in the cavity in the cylinder head to deliverfuel in a direction towards the piston, the piston having a top facedirected toward the cylinder head; a bowl in the top face of the pistonpositioned so that throughout at least a final half of the compressionstroke of the piston, the bowl is located in a path of delivery of thefuel from the nozzle so that fuel will be delivered into the bowl, and arim extends about the periphery of the bowl upstanding from the topsurface of the piston, wherein an external peripheral surface of the rimis inclined upwardly and inwardly at an angle of 30° to 50° to the axisof the bowl.
 3. An internal combustion engine, comprising:a cylinder; acylinder head at one end of the cylinder; a piston mounted forreciprocation in the cylinder; a cavity in the cylinder head; a fuelinjector nozzle located in the cavity in the cylinder head to deliverfuel in a direction towards the piston, the piston having a top facedirected toward the cylinder head; a bowl in the top face of the pistonpositioned so that throughout at least a final half of the compressionstroke of the piston, the bowl is located in a path of delivery of thefuel from the nozzle so that fuel will be delivered into the bowl, and arim extends about the periphery of the bowl upstanding from the topsurface of the piston, wherein the bowl and rim have a total depth ofabout 5 to 8 mm.
 4. An internal combustion engine, comprising:acylinder; a cylinder head at one end of the cylinder; a piston mountedfor reciprocation in the cylinder; a cavity in the cylinder head; a fuelinjector nozzle located in the cavity in the cylinder head to deliverfuel in a direction towards the piston, the piston having a top facedirected toward the cylinder head; a bowl in the top face of the pistonpositioned so that throughout at least a final half of the compressionstroke of the piston, the bowl is located in a path of delivery of thefuel from the nozzle so that fuel will be delivered into the bowl, and arim extends about the periphery of the bowl upstanding from the topsurface of the piston, wherein the depth of the bowl in the piston isabout half the total depth of the bowl and rim.
 5. An internalcombustion engine comprising:a cylinder; a cylinder head at one end ofthe cylinder; a piston mounted for reciprocation in the cylinder; acavity in the cylinder head; spark means mounted in the cylinder forproviding an ignition spark in the cavity; a fuel injector nozzlelocated in the cavity in the cylinder head to deliver fuel in adirection towards the piston, the piston having a top face directedtoward the cylinder head; a bowl in the top face of the pistonpositioned so that throughout at least a final half of the compressionstroke of the piston, the bowl is located in a path of delivery of thefuel from the nozzle so that fuel will be delivered into the bowl, aninternal surface of said bowl having a flat bottom configuration whichdirects the fuel along a path substantially reverse to a direction ofthe incoming fuel, wherein a rim upstanding from the top face of thepiston and shaped to create an air flow in said reverse direction of thefuel flow extends about a periphery of the bowl to substantiallysurround and thereby contain said reverse fuel flow during said finalhalf of the compression stroke.
 6. An engine as claimed in claim 5,wherein the bowl is of substantially circular cross-section and the axisof the bowl is substantially aligned with the axis of the fuel injectornozzle.
 7. An engine as claimed in claim 5, wherein said bowl and rimhave respective internal peripheral surfaces arranged contiguous.
 8. Anengine as claimed in claim 5, wherein the external peripheral surface ofthe rim is of a concave contour.
 9. An engine as claimed in claim 5,wherein the longitudinal length of the cavity is between 0.5 and 0.8 ofthe diameter of the cylinder.
 10. An engine as claimed in claim 5,wherein the substantially straight side edges of the cavity are spacedapart a distance of 0.35 to 0.65 of the diameter of the cylinder.
 11. Anengine as claimed in claim 5, wherein the cavity has substantiallyparallel opposite side walls extending from said substantially straightside edges.
 12. An engine as claimed in claim 5, wherein the ratio ofcavity length int he longitudinal direction to the cavity maximum depthis in the range from 1 to
 3. 13. An engine as claimed in claim 5,wherein the maximum depth of the cavity is 0.35 to 0.45 of the cylinderdiameter.
 14. An engine as claimed in claim 5, being a two stroke cycleengine having inlet and exhaust ports in the cylinder wall with at leastone inlet port at a substantially opposite location in the cylinder tothe exhaust port, and the cavity in the cylinder head extends in agenerally diametral direction from adjacent the cylinder wall at alocation opposite the exhaust port, the cavity having substantiallystraight side edges in the diametral direction of extent thereof, thelongitudinal length of the cavity in said direction being less than thediameter of the cylinder.
 15. An engine as claimed in claim 14, whereinthe bowl in the piston is offset with respect to the axis of thecylinder in the diametral direction of extent of the cavity in thecylinder head, so the bowl is generally aligned below the cavity.
 16. Anengine as claimed in claim 5, wherein the cavity in the cylinder headprogressively increases in depth from the one end adjacent the cylinderwall to a location of maximum depth and thereafter progressivelydecreases in depth to the other end of the cavity, whereby the base ofthe cavity presents a generally concave surface to the piston that willinduce gas entering the cavity at said one end to establish a rotationalmotion about an axis transverse to the longitudinal direction of thecavity as the gas passes through the cavity, whereby the gas uponleaving the cavity is induced to move towards said one end of thecavity.
 17. An engine as claimed in claim 16, wherein progressively thelocation of maximum depth of the cavity is spaced from said cylinderwall a distance between 0.25 and 0.5 of the cylinder diameter, and saidmaximum depth being 0.25 to 0.55 of the diameter of the cylinder, whenmeasured from the diametral plane of that face of the cylinder headdirected towards the piston.
 18. An engine as claimed in claim 5,wherein said bowl is positioned in the piston so that throughoutsubstantially the whole of the compression stroke of the piston, thebowl is located in the delivery path of the fuel from the nozzle.
 19. Anengine as claimed in claim 5 or 18, wherein the top surface of the rimis contoured to sweep upwardly from the top face of the piston to thetop edge of the piston.